Bart, get out! I'm piss!

From Bibliotheca Anonoma
The original comic that started the meme

Badly drawn pornography involving Bart and Lisa Simpson. Bart walks in on Lisa while she is urinating, to which she shouts "Bart, get out! I'm piss!" It is a popular catch-phrase, and is normally used when comics (where this comic came from) are flooded in /b/.